In outlining his ideas for immigration reform Wednesday evening, Donald Trump said that blocking funding for sanctuary cities would be one of the central parts of his plan.

“We will end the sanctuary cities that have resulted in so many needless deaths,” Trump pledged.

“Cities that refuse to cooperate with federal authorities will not receive taxpayer dollars, and we will work with congress to pass legislation to protect those jurisdictions that do assist federal authorities.”

According to the Center for Immigration Studies there are about 300 jurisdictions that have been identified by ICE as having policies that are “non-cooperative and obstructs immigration enforcement.”

“It is a notice to another law enforcement agency that ICE intends to assume custody of an alien and includes information on the alien's previous criminal history, immigration violations, and potential threat to public safety or security,” Vaughan described.

According to the Associated Press, supporters Sanctuary Cities say the detainers can unfairly target innocent immigrants and hurt relations between immigrant communities and law enforcement authorities.

In attacking Hillary Clinton and President Obama, Trump accused the two of standing behind sanctuary cities.

“President Obama and Hillary Clinton support Sanctuary Cities, they support catch-and-release on the border, they support visa overstays, they support the release of dangerous criminals from detention – and they support unconstitutional executive amnesty,” Trump said.

The Obama administration “has criticized cities like San Francisco for not cooperating fully with immigration officials, but has tried to work with them through a program that would allow federal officials to be notified before a serious criminal is released,” The Associated Press reported.

The Priority Enforcement Program was established in 2014 and replaced the Secure Communities Program, according the Immigration and Customs Enforcement.

The new program differs from its’ predecessor by “targeting individuals convicted of significant criminal offenses or who otherwise pose a threat to public safety.”

Citing ICE Statistics, Vaughan wrote that “since the Obama administration implemented the new Priority Enforcement Program in July 2015 restricting ICE use of detainers, the number of rejected detainers has declined.”

“However, the number of detainers issued by ICE also has declined in 2016, so it is not clear if the new policies are a factor,” Vaughan noted.

“It is apparent that most of the sanctuary policies remain in place, raising concerns that the Priority Enforcement Program has failed as a response to the sanctuary problem, and has simply resulted in fewer criminal aliens being deported.”

Trump’s proposal would withhold federal funding for sanctuary cities as a penalty.

According to Ira Mehlman, Media Director for the Federation for American Immigration Reform, the amount of federal funding that these sanctuary cities is receiving depends on the size of the jurisdiction.

“For a city like Los Angeles,” Mehlman explained it would be a “significant amount.”

Regardless of how much federal funding a city is receiving, it can still impact a locality's bottom line.

“It can make a huge difference, even if it represents a relatively modest portion of their overall budgets,” Mehlman said.

Describing the feasibility of cutting this funding, Mehlman explained that this is a commonly used tactic.

“Obviously Congress appropriates that money and ear-marks it,” Mehlman said, but “the federal government has used the threat of cutting off federal funds to convince state and local governments to do all sorts of things.”

“It’s a very common practice,” Mehlman said, explaining how every state and local government is looking for every dollar they can get their hands on.

“This can make a significant difference,” Mehlman said.

Part of Trump’s promise to crack down on sanctuary cities is asking Congress to pass Kate’s Law on his first day in office. The legislation is named for Kate Steinle, a San Francisco woman who was shot and killed by an undocumented immigrant in the summer of 2015.

As Trump stated in his speech, the man who shot Steinle had been deported five times. He had also been released from jail months earlier, a fact that helped fuel a fierce debate over sanctuary cities in the wake of Steinle’s death.

Kate’s proposed increasing the penalties applicable to illegal immigrants after being removed from the United States.

The bill was proposed by Louisiana Sen. David Vitter and would punish jurisdictions that don't share immigration information or cooperate with federal requests, blocking them from receiving certain grants and funds.

"Rather than reward cities, we must start enforcing our current immigration laws and strengthen our borders to keep Americans here safe at home," Vitter told the Associated Press at the time.

Senate Democrats ended up blocking the bill with a 54-45 vote. Had it passed, The White House threatened to veto the measure, according to The Associated Press.

While Trump did not specify what legislation he would be working with Congress on to strip funding from Sanctuary cities and protect those that comply, his speech made one thing clear: such legislation would not face a veto threat in a Trump White House.